Removal of unwanted hair is a common cosmetic concern. Traditional methods of hair removal which include shaving, waxing, tweezing, depilatory creams and electrolysis, result however only in temporary hair removal, can be painful and rely on techniques requiring multiple treatments.
Photoepilation (also known as light-based hair removal) is a non-surgical cosmetic method based on light technology to remove unwanted hairs and slow down or prevent their regeneration. The thermal effects generated during irradiation with a light source that can be either a laser or an intense pulsed light (IPL) are principally responsible for hair follicle alteration and damage. Thermal damage to the hair follicle is the consequence of light absorption by endogenous melanin. Results therefore are dependent on hair pigmentation, quantity of melanin present in the hair and the ratio between melanin concentration in the hair bulb and in the epidermis.
Since these approaches rely on the natural color (absorption) contrast between the hair and the skin, they consequently fail for fair hair (white, blond and auburn) and even in the ideal configuration of dark hair on pale skin the required light intensities are responsible for local skin injuries that can be long-lasting or even permanent. Methods have been developed to counteract these disadvantages, including the use of exogenous chromophores to increase the light absorption efficiency of the hair follicle in comparison with endogenous melanin absorption, and thus increase the safety of the procedure by reducing the laser power needed.
In this sense one of the recent approaches for hair removal is described in U.S. Pat. No. 6,287,549. Photosensitizers are encapsulated in a microsphere of size between 3-10 microns in diameter to obtain specific follicular targeting before laser irradiation. In general, these photosensitizers are exogenous chromophores and when used in conjunction with light renders them active. However, this approach presents among others the important drawback that the delivery of microspheres is non-specific, insofar as they do not only target the hair follicle, but can also be found in other skin areas such as the channels of sweat glands where they can cause damage and/or skin discoloration upon irradiation.
US2003/0059386 discloses topical compositions able to target hair follicles after initial hair removal from the follicles. Said compositions are able to selectively introduce photosensitizers or sonosensitizers compounds encapsulated in microparticles of size ranges between 1 to 70 microns. The composition is topically applied on skin, thereafter the compounds are released from the microparticles into the hair follicle and surrounding tissues, and then the skin is treated with electromagnetic or ultrasonic radiation. The compounds are thus activated, altering and/or killing cells responsible for hair growth. This hair removal method is however not completely satisfactory since microparticles reach surrounding tissues, and this causes upon irradiation, significant unwanted damage. Besides high radiation energies to activate compounds are needed which causes irritation and erythema.
Thus, in view of the above stated examples there remains the need in the state of the art to provide alternative and efficient photoepilation methods and compositions, which overcome all or at least part of the aforementioned drawbacks.
The solution provided by this invention is based on the fact that the inventors have discovered that resonant nanoparticles (NP) coated with at least a chemical compound applied on skin can be designed to target the hair follicle and attach to the hair cuticle via electrostatic interactions or to specifically bind to a target molecule present on the basal cells near the hair root. Once the coated resonant NP have attached to the hair, they can be locally and efficiently heated upon illumination due to their resonant properties, (with a reduced intensity/energy radiation) and destroy the hair without damaging surrounding tissues and/or glands. This approach presents the important advantage that the radiation needed for the present photoepilation method can then be of reduced intensity compared to conventional photoepilation methods.